The present invention relates to a frictional jointing, and in particular to a frictional joining technology in which different kinds of metal members that lap over are joined.
Members made of aluminum or aluminum alloy (hereinafter, referred to as “aluminum” simply) have been recently used as a body panel and the like of many automotive vehicles to reduce a vehicle weight. Accordingly, the joining of different kinds of metal members, such as aluminum and iron, or aluminum and steel, has been increasing. The frictional joining is also known as such a joining method. In this frictional joining, there is provided a work that is comprised of a first metal member, for example, that is made of aluminum alloy and a second metal member, for example, that is made of steel and has a higher melting point than the first metal member, which lap over. Then, a rotational tool is pushed against this work from a side of the first metal member, the first metal member is softened and made in a plastic flow state by a frictional heat generated via the rotational and pressing operation of the rotational tool, and so both metal members are joined in a solid state (joining in the solid state without melting) under a specified temperature that is lower than the melting point of the metal members.
Japanese Patent Application Laid-Open No. 2003-245782 discloses a certain spot joining technology. Herein, the rotational tool under rotation having its pin portion and shoulder portion is pushed against the work comprised of plural metal members at the high rotational speed and with the large pressing force at the initial stage in which only the pin portion contacts the work, so the generation of the frictional heat can be promoted. Meanwhile, the rotational tool under rotation is pushed at the low rotational speed and with the small pressing force at the terminal stage in which both the pin portion and the shoulder portion contact the work, so the proper agitation of the softened portion of the work can be promoted.
Also, the applicant has applied for a patent relating to the improved spot joining method and apparatus of the metal members in which the rotational tool is pushed with the stepwise increased pressing force to ensure the proper positioning of rotation by the pin portion of the rotational tool (U.S. patent application Ser. No. 11/000,063).
Meanwhile, in the case where different kinds of metal members, such as aluminum and steel, are joined, it is preferable in order to ensure the high joining strength that new uncovered surfaces of the metal members are exposed by pushing out the zinc plating layer existing on the joining boundary face from the joining portion of the members or by destroying the oxidation film. In order to do so, the metal member into which the rotational tool comes needs to be softened sufficiently and the plastic flow is generated. Here, in order to properly generate the plastic flow, it is necessary to promptly increase the heat generated at the joining portion to a temperature that can soften the metal member, and then to maintain the increased temperature for a while so that the continuous plastic flow of the metal member can be ensured. Herein, the invention of the above-described patent application is appropriate in order to promptly increase the temperature of the joining portion to the temperature to soften the metal member, because the considerably high pressing pressure is required. However, there are following concerns.
Namely, a certain period of time of agitation by the rotational tool under a relatively high pressing force at the terminal stage with both the pin portion and the shoulder portion of the rotational tool pushed into the metal member to generate the proper plastic flow at the metal member would cause an improper situation in which the rotational tool comes into the upper aluminum plate too deeply, so that the thickness of part of the aluminum existing between the tip of the rotational tool and the lower steel plate (remaining thickness) becomes too thin. Eventually, there would occur a situation in which the aluminum plate has been torn off. As a result, an aluminum loss would happen at the joining portion, and thus there would occur problems of galvanic corrosion or joining-strength decrease due to a difference in electric potential between the aluminum and the steel at this aluminum-loss portion. Also, the torn-off aluminum is attached to the rotational tool, which would prevent the rotational tool from properly operating for joining at subsequent joining portions. Further, the rotational tool penetrates the upper aluminum plate, and reaches the joining boundary face, hitting against the lower steel plate. As a result, the tip of the pin portion of the rotational tool would be worn improperly.